Synoptic Characteristics of Linear Mesoscale Convective System in Shandong Province
The synoptic characteristics of LMCS (linear mesoscale convective system) were analyzed based on 27 selected LMCSs affecting Shandong from the 2012-2016 radar data. The filter conditions are as below: the contiguous or quasi-contiguous echo band larger than 40 dBz is over 100 km and lasts for at least 1 h, the 35 dBz echo embedded with the 40 dBz echo is strictly contiguous, the linear or quasi-linear convection area shares a common leading edge, and the maximum echo intensity is over 50 dBz. The characteristics of the study are concluded as follows. The LMCS affected Shandong has a high frequency in August, and the formation time is concentrated in the dusk to the first half of the night, lifespan is generally 1-2 h, and most of them have characteristics of backward propagation. Most of the initial convective cells are generated in Hebei and generally move eastward by southward. Most of the LMCSs are in a northeast-southwest trend, and the scale is between 100 and 200 km. Three types of synoptic models of LMCS formation are constructed, including forward-tilting trough, backward-tilting trough and cold vortex. The warm temperature ridge or warm center at 850 hPa is the important feature of LMCS formation. The mid-level jet stream at 500 hPa in cold vortex and forward-tilting trough category, and the low-level jet stream below 700 hPa in backward-tilting trough category all play very important roles in the LMCS formation. LMCS constantly occurs when 850 hPa specific humidity is greater than 8 g·kg-1 and the Lift Index (LI) and Showalter Index (SI) are both negative. The probability of LMCS occurrence is up to 80% when T850-500 >25℃ and CAPE >1 〖KG-*5〗000 J·kg-1 with small CIN. When LMCS appears, it is usually accompanied by short-duration heavy precipitation, 70.4% LMCS contributes to disasters of thunderstorm gale, hail and severe precipitation. The hail and gale require higher stratification instability than short-duration severe precipitation. When only the short-duration heavy precipitation occurs, the heights of 0℃ and -20℃ layers are significantly higher than those when hail and gale occur.